A Guidebook to Mechanism in Organic Chemistry

Carbanions and Their Reactions

H

t

R—CH^-N

O

R—CH*-N

9°

o

fc

R—CH=N

O^0 J

HO r o~s o

e

i

R—CH^C—R -> CHJ-C—R ~ R—CH=C—RJ

H F

t / B: 0

R—G»-C->-F -* R—CH—GF 3

\

F

The smaller fK„ and hence greater stability of the carbanion, of

acetylacetone (VIII) compared with ethyl acetoacetate (VII), and of

the latter compared with diethyl malonate (VI) arises from the car­

bonyl group in acetone being more effective at electron-withdrawal

and delocalisation, than the carbonyl group in an ester. This springs

from the occurrence in the latter of

Ml p.

R—C-I-OEt

which lowers the effectiveness of the^>C=0 group at withdrawing

electrons from the rest of the molecule. The decreasing activating

effect of -COY on going -COH-* -COR->- -COOR-*-

-CO-NH 2 -»- -CO-O® being due to Y becoming more electron-

donating as the series is traversed.

An interesting carbanion.'the cyclopentadienyl anion (IX)

e

(IX)

well. Thus a carbonyl or a nitro-group will be more effective than,

for example, NR 3 , CF 3 , etc.: